One kind of hydraulically controlled variable deflection roll is disclosed by the 1964 Kusters et al U.S. Pat. No. 3,131,625.
This roll comprises a rotative shell forming an external work-rolling surface and a cylindrical inside. A fixed shaft extends through the shell's inside with radial clearance permitting independent transverse deflection of the roll and shaft. The shaft has a longitudinally extending series of radially extending cylinders formed by blind bores in the shaft. Each cylinder contains a single reciprocating piston for applying the roll deflection force between the shaft and the shell via a bearing shoe bearing on the shell's cylindrical inside.
When in use with the roll rotating against a counterroll to form a nip through which flat work can be rolled, hydraulic pressure is introduced uniformly to the cylinders so their pistons force the shoes against the shell's inside to control the shell's contour. The reaction is carried by the shaft which functions as a beam and consequently bends under the beam stress it receives, whether the roll's controlled contour is straight or curved.
Excepting for the cylindrical bores and relatively small hydraulic liquid feed passages to the various cylinders, the fixed shaft is made of solid metal. Its maximum diameter is limited by the need for clearance between it and the shell's inside. The radial extent or depth of the bores affect the beam strength and stiffness of the shaft, so it is desirable to have the bores as short or shallow as possible. It has been necessary to make the cylinders formed by the bores, and their pistons, of large diameter to provide adequate roll deflection force without using abnormal hydraulic pressure.
It is common rule that to avoid a piston tilting or canting in its cylinder, the piston must have a guided length 1.5 times its diameter, necessarily requiring its cylinder to be substantially longer to accommodate the piston's working stroke. It follows that in the case of the described roll the bores forming the cylinders must extend undesirably far into the fixed shaft in its radial direction with a consequent undesirable reduction in the shaft's beam strength and stiffness.
The above rule can be satisfied without using such deep bores by using cylinders and pistons of much smaller diameter than has been usually contemplated, because it is possible to provide hydraulic pressures high enough for such arrangements to have the force necessary for them to be substituted for the larger diameter cylinders and pistons customarily proposed. Although abnormal pressures would be required, such pressures are technically attainable.
The above possibility, if reduced to practice, would require much better sealing between the pistons and their cylinders than is required when the larger diameters are used. As previously noted, the fixed shaft which functions as a beam carrying the reaction to the force applied to the shoes, bends under the beam stress it receives. The beam deflection or bending can be very substantial in some instances. For example, with large controlled deflection rolls having the dimensions commonly used to calender paper webs in a paper mill, the roll length may be in the area of 8 m long and the fixed shaft at its central portion may deflect as much as 20 mm under the beam stress it receives, and this is in the case of rolls designed with side seals so that the hydraulic pressure is applied to the working side of the fixed shaft throughout 180.degree. of its circumference. The shaft can be made of solid steel and with a diameter that almost completely fills the inside of the rotative shell receiving the pressure. With such deflections, which would be exaggerated if the shaft were to be bored to form even very small diameter cylinders, the small cylindrical shapes would distort from true cylinders to slightly elliptical shapes preventing the necessary tight sealing between the pistons and the cylinders required by the high hydraulic pressures that would be required with the small diameter elements.